I understand the physical aspect of what you write. What does not add up is your ip addressing as well as having the server and pcs raw on the internet. What you infer is the dongles can do a private wifi network as well as internet connection. I have never seen a 3g dongle that fits those specs.
I have seen using a 3g dongle to share internet with others
I have seen vpn access to a private network via 3G dongle.
I have seen 3g dongle in a 3g router.
Do you have a link to those 3G dongles you used?
"If you want the internet on any of the 5 computers, you need to disconnect from the private network on the ppp adapter and then use the ethernet."
Two things, one is the OP clearly states computers are far apart so using the lan port is not an option. Two is if the lan port was a option you would only use the 3G for internet access. You can do both lan and internet at the same time. Trick is no gateway entry on the lan zettings. So why, in your case, would you use the 3g to create a private network when you have the lan available?
See why what you write isn't adding up for me?